Transcript Slide 1

Brownfield
Briefing
“VOCs and
Ground gases
“Conference,
London
Gas Protection- What is
needed?
Selected case studies of some common
problems and pragmatic solutions
18 November 2009
Jo Strange MEng, C Eng C Env, MICE, MIEnvSci, SiLC
Regional Director, Card Geotechnics Ltd
Introduction
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Ground gases
“Conference,
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Despite a plethora of
guidance, there are still a
number of issues which are
commonly seen which lead
to either poor or over
conservative design of gas
protection measures for
buildings.
The following case studies
identify some of those
issues and how they were
overcome to provide a
pragmatic solution.
Typical Basic Issues
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•Much guidance- but lack of
understanding,
•Poor SI specification eg installation details
•Poor quality data- data acquired during
poor monitoring conditions, missing flow
data
•Lack of site model or understanding of
gassing regime
•Omitted gas protection measures
•Over design or poor design of gas
protection measures
Misinterpretation of Data:
Example – Residential
Development South East London
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2.
3.
4.
Site geology comprises typically non-organic
Made Ground (gravelly clay or sand & brick)
over Alluvium, River Terrace Deposits and
Lambeth Group
Groundwater in borehole 6.1mbgl
Shallow well response zone sealed within MG
and Alluvium (Max CH4 =0.1%v/v, Max CO2=
3.4% Max Flow 0.5 l/hr )
Deep well response zone sealed within River
Terrace Deposits (9-13mbgl) (Max CO2=
3.9%v/v, Max CH4=3.4% Max Flow 0.1 l/hr
…cont’d
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ORIGINAL CONCLUSION
Gasprotection
caused by anaerobic
within closed
Gas
requiredreaction
to Characteristic
void
– typified
by CH4/CO
no flow,
in on
2 with665),
Situation
2(CS2)_
(CIRIA
based
waterlogged well with no gassing source.
max CO2 in deep well
Not a sustained source- gas resulting from
borehole installation
BUT……
REVISED CONCLUSION
Based
well construction,
where(CS
is 1gas
No gason
protection
measures required
source in deep well ?
applies)
Result
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Reduction in requirements for specific
gas protection measures.
Therefore…..
Cost and construction time saving for
Client.
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Interpretation of Anomalous data:
Example- Residential
Development in vicinity of closed
landfill site, Hampshire
London
18 November 2009
• Site being developed on farm land close to closed
landfill with active extraction system.
• Farm land between landfill and development to be
handed over to local authority as Public Open
Space, as part of planning negotiations.
• Unlicensed landfilling identified outside landfill
boundary.
• Local authority required robust reassurance that
appropriate gas protection measures were proposed.
Cont’d
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• Numerous investigations undertaken and gas
protection measures to CS 3 proposed by
developer’s consultant.
• Local Authority queried anomalous data,
which included:
Extremely high flows on occasions to south
of site, with little methane and carbon
dioxide, and IN SAME BOREHOLES,
negligible flow and elevated methane and
carbon dioxide.
• Local Authority refused to accept proposed
design until anomalies explained.
Cont’d
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• Geology confirmed as Wittering
Formation of the Bracklesham Beds
(typically silt and CLAY) overlying
Whitecliff Sand
• Strata dip towards the south
• Groundwater was at depth, but
anecdotal evidence of large local
fluctuations in groundwater level.
Clarification of Ground Gas
Regime
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• Gas was migrating from unlicensed extension to landfillconcentrations decreasing with increasing distance from
landfill
• Migration occurred effectively within the Whitecliff Sands
• Gases confined below the Wittering Formation
• Gases pressurised by fluctuating groundwater, causing high
flow rates, increasing to south of site due to ‘funnelling effect’.
• Radiocarbon dating showed gas in Whitecliff Sand to be from
a modern anthropogenic source, ie landfill.
• Similar testing showed gas in Wittering Clay to south of site
from a geological source i.e. derived from biological reactions
in the soil. (Gas generation caused when risen groundwater
sealed base of monitoring well, hence zero associated flows)
Ground Gas Model
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Solution
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• Confirmation of a minimum thickness of clay
mantle between the base of the foundations and the
underlying sand (requiring make up in some areas
from a borrow pit on site)
• Gas protection measures to CS2 to be installed
continuous, robust gas membrane incorporated
within the floor slab construction to mitigate any
residual gas.
• Penetration of the membrane limited by design service entries were limited. All service
penetrations were sealed through the membrane
• Passively vented under floor voids
• Monitoring to confirm gas regime above and below
clay mantle.
Result
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• Local Authority agreed gas regime and
revised proposed gas protection
measures
• Monitoring confirmed the different gas
regimes above and below the clay
mantle
• Development completed and occupied
Missing Gas Protection Measures
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• This happens not infrequently
• Occasionally due to architect/structural engineer
being unaware of requirement for gas protection
measures and nothing shown on construction
drawings
• More often due to lack of attention to detail by
contractor operative under pressure
• Often missed when no requirement for
environmental engineer to be present on site to
observe/verify incorporation of gas protection
measure.
Solution usually depends on when the error is
found!
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Use of Risk Assessment:
Example: Mixed
residential/commercial
development, East London
• Site located on ex-industrial site, with a buried oil
tank.
• Limited gas monitoring pre-construction indicated
worst case CS4 conditions, strictly requiring a gas
resistant membrane and underslab passive
ventilation.
• During construction, the passive ventilation was
omitted.
• CARD commissioned to undertake a risk
assessment to confirm whether development was
safe without ventilation for submission to Planning
Authority.
Site Conditions
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• Review of data showed that gas
concentrations assessed for individual
blocks generally fell within CS1. Data for
one block fell within CS2. (Higher category
was due to limited time frame for
monitoring.)
• Since design of development, remediation
works completed to remove buried fuel tank
and surrounding impacted soil in location
where worst gas conditions measured.
• Up to 2m of Made Ground had been
removed from site during archaeological dig
and replaced with granular fill.
Table 5.1–Classification of risk, CIRIA 152, 1995.
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Risk Assessment
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• Risk assessment carried out using CIRIA
152 fault tree analysis for a range of
scenarios e.g. explosion or asphyxiation
• Site zoned according to location specific
worst case data.
• Account taken of changes to ground
conditions.
• Account taken of construction
methodologies ie piled raft foundations,
membrane installed and building dimensions
Figure 5.2–Classification of risk, CIRIA report 152
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Result
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• the highest risk of explosion is in the
smallest confined space
• the highest risk of asphyxiation relates to the
long term exposure within the small
bedroom.
• All of the scenarios calculated probability of
risk scenario as not exceeding 1 in 1 million
(i.e. 1x10-6), (the accepted tolerable level.)
• Able to reduce required gas protection to use
of waterproofing membrane only in worst
case zone
• satisfied local authority.
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Retrofit of Gas Protection
Measures:
Example- Commercial building,
Portsmouth
• Due to technical problems encountered
during construction of the lift pits, the
membrane was not installed prior to casting
the concrete.
• This came to light when verification
inspections were requested by the contractor.
• It was not possible to install gas resistant
membranes on the inside of the lift pit due to
the lack of spatial tolerances.
Solution
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Given the site constraints, a liquid
membrane solution was recommended,
(Tecroc Liquid Vapour Membrane, LVM)
Four coats of the LVM (equivalent to
specified sheet membrane) was applied to all
internal surfaces of the lift pit, tying into the
gas membrane at ground level, to achieve
the required gas permeability standards.
This approach was approved by Portsmouth
City Council Contaminated Land team.
Liquid membrane – In situ
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A clever touch:
The LVM comes in two colours, black
and white, so that each layer can be
clearly seen as it covered by the next
layer in the alternative colour.
Brownfield
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Poor design:
Example- Commercial
development in Hampshire
• Building constructed adjacent to
landfill site incorporating ‘gas
protection measures’ was at
commissioning stage.
• Gas concentration of between 25% and
113% of LEL in switch cupboards after
New Year holiday
• Request from client to advise on
building occupancy. (CARD not involved in
investigation or design)
Site Assessment
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•
•
18 November 2009
•
•
•
Investigation information indicated 7.05
l/hr per sq metre
Identified concentrations of 47.7%
methane and 24% carbon dioxide.
Concentrations changing over tidal cycle.
Design used Somtube connected to
manifold and cowls on chimneys on one
side.
Inlet at low level on side nearest to landfill.
Design drawing
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What did we find?
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• Design did not permit the dilution
approach under low flow or no flow
conditions.
• Identified approximately 150 unsealed
service entry points.
• Degree of protection relied on quality
of pre cast slab and membrane.
Not quite a diluting ventilation
layer….
Brownfield
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Ground gases
“Conference,
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18 November 2009
Solution
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• Provide temporary seals to all service ducts
and drainage.
• Establish regular monitoring regime
• Determine key areas for permanent methane
monitoring
– False floors
– Switch rooms
• Carry out rigorous survey of service ducts
and provide internal two phase gas seal.
• Continue to monitor
Result
Brownfield
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“VOCs and
Ground gases
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London
18 November 2009
• Site made safe for occupancy
• Site management scheme set up with
permanent monitoring of critical zones
• Client confidence restored
Thank you for listening
Brownfield
Briefing
“VOCs and
Ground gases
“Conference,
London
18 November 2009